The present invention relates generally to wireless radio telecommunication systems. More specifically, the invention relates to a method of reducing the interference generated by mobile station transmitting at an inappropriately high power level following a hard handoff.
In a basic cellular telecommunication system, as illustrated in FIG. 1, a system controller is linked to a network of base stations by a series of digital transmission links 115. The base stations are geographically dispersed to form an area of coverage for the system. Each base station (BS) is designated to cover a specified area, known as a cell, in which a two way radio communication connection can take place between a mobile station and the BS in the associated cell. In this simplified exemplary depiction, only two base stations are shown but in practice, a substantial multiplicity of base stations will form the functional coverage area for the system. It is understood by those skilled in the art that other components and devices are typically included in the system that are not shown in the exemplary illustration. In general, as the MS moves throughout the network, communications are maintained with the network by transferring the connection to a neighboring base station in an event referred to as a handoff. For simplicity, the term mobile station will henceforth be referred to as the mobile.
In telecommunication systems operating in accordance with core division multiple access (CDMA), mobiles may simultaneously communicate with more than one BS prior to a handoff from an originating BS to a neighboring BS. This is referred to in the art as xe2x80x9csoft handoffxe2x80x9d in that the mobile will commence communication with the neighboring BS before terminating communication with the originating BS. This xe2x80x9cmake before breakxe2x80x9d procedure is made possible by operating all traffic on a common spread spectrum waveform frequency. A variant of the soft handoff is what is referred to as xe2x80x9csofter handoffxe2x80x9d in which the mobile simultaneously communicates with multiple sectors of the same BS. There are several advantages associated with soft handoffs such as reduced risk of dropped calls, no interruption in speech upon handoff, increased gain in downlink signal-to-noise ratio, and greater protection from log normal and multi-path fading since, on average, the convergence from the effects of fading or multi-paths do not occur at the same time.
In an exemplary CDMA system, handoff decisions are typically based on the detection by the mobile of the signal strength of pilot signals transmitted by neighboring base stations. The pilot signals are distinguished by a pseudonoise sequence (PN) such that the mobile is able to determine and allocate the base station within a distinct classification set. By way of example, the sets include an Active Set which is a set of base stations that the mobile is actively communicating with, a Candidate Set which is a set of base stations that have a pilot strength that are sufficient for communications based on system parameters set by the base station, and a Neighbor Set which is a set of base stations in the area that have a pilot strength indicating the potential for sufficient communication with the mobile. The base station""s classification within a set may be modified in accordance with the received pilot signal strength by the mobile. Handoff decisions are then made by the system controller which are typically based in part on the reported pilot signal strength.
Another type of handoff that occurs in CDMA systems is a xe2x80x9chard handoffxe2x80x9d which is a handoff that takes place, for example, between two frequencies or when the base stations are not suitably synchronized for a soft handoff. This type of handoff is often characterized as xe2x80x9cbreak before makexe2x80x9d since communication on a first frequency is terminated before communication is established on a second frequency. Hard handoffs occurring within the same cell are referred to as intra-cell hard handoffs and those occurring between cells are referred to as inter-cell hard handoffs. Hard handoffs typically occur in situations where vendor equipment limitations preclude performing soft handoffs such as, for example, layer changes for moving mobiles, mode switches e.g. in dual mode systems, switching between operator networks, and resource allocation issues that require intra-frequency hard handoffs.
The disadvantages of performing hard handoffs include an increased probability of dropped calls, speech interruption, and loss of soft handoff gain. Furthermore, hard handoffs may negatively affect the performance and, the overall quality of the connections within the system. By way of example, a notable problem within an exemplary CDMA system is regulation of the transmit power level emitted from a mobile immediately following a hard handoff. This may happen, when a mobile at the edge of a coverage area is communicating with its serving BS and is thereby transmitting at high power. The mobile in this situation can be significantly closer to the neighboring BS thus a handoff at this point at the current high power level will create substantial interference in the new cell since a relatively low power is adequate to sustain communication in the new cell. The situation can also be exacerbated by shifting cell borders due to cell shrinkage/expansion resulting from an increased/decreased number of active mobiles within the serving cell. The tendency for cells to shrink and expand in relation to capacity is known in the art as cell breathing and is a notable factor in the system layout design.
Following a hard handoff, the mobile cannot know the appropriate transmit power level since the neighboring BS is not able to control the mobile""s power level as long as it is still connected to the original serving BS. As a result, the mobile may enter the new cell at an excessively high power level thus generating a burst of uplink interference which thereby degrades the connection quality of other mobiles sharing the same frequency. Once in the new cell, the power level is gradually adjusted by a power control algorithm in order to bring mobile within suitable, operating limits.
One solution that has been proposed is to anticipate a reduction in power following the hard handoff. In for example CDMA networks operating in accordance with IS-95B, a cell parameter INIT_PWR is used together with the power control algorithm in an attempt to reduce the power level to the appropriate required power level in the new cell. The cell parameter INIT_PWR is implemented for use in the reduction of the power level by immediately reducing the power following a hard handoff. This reduction in the power level is referred to as the power margin, and is typically on the order of 3 dBm but may be set to any level by individual operators. The implementation of the INIT_PWR parameter in IS-95A was not originally intended for power control issues associated with hard handoffs but instead its specified use was for the reduction of mobile transmit power upon first access for which the appropriate power level is not known.
A disadvantage of using the INIT_PWR parameter in this way is that the specified reduction may not be suitable for all operation conditions since the power level required for sufficient communication may vary over time, for example, due to cell breathing. This is illustrated by depictions of the power level activity throughout the day for individual cells. For example, daytime mobile transmission power is typically higher prior to a handoff due to cell breathing from the increased interference from more operating mobiles in the system and from other interference sources. In contrast, nighttime operation typically shows that transmission power levels are relatively constant before and after a handoff event, thus an obligatory drop in transmission power may unnecessarily degrade signal quality. Since the magnitude of the required reduction may vary throughout the day, the relative inflexibility of the prior solution, e.g. with a fixed INIT_PWR, may not always result in optimum operating levels.
In view of the foregoing, it is an objective of the present invention to provide for an improved power regulation technique for mobiles following hard handoffs.
Briefly described, and in accordance with an embodiment thereof, the invention discloses a method of reducing the power level transmitted by a mobile following a hard handoff in a code division multiple access based system. The method includes building a database containing data associated with the transmit power levels from a statistical collection of hard handoff events. A profile of the data is developed in order to determine an adequate power margin to be applied to mobiles that perform hard handoffs between specific cells or specific locations. The power margin can then be applied, scaled down, or removed based on the current loading on the system or other relevant factors.
The present invention provides an effective means for reducing the amount of interference generated by mobiles transmitting at unnecessarily high power levels thus improving the overall performance and quality in the system. These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings.